El Túnel is a dark story about a deranged porteño painter, Juan Pablo Castel, and his obsession with a woman. The title of the story comes from the tunnel of loneliness and isolation that Castel sees himself at the end to be in.
The obsession begins on 21 September 1946 when Castel sees a woman at an exhibition of his work focusing on one small detail of one painting painting of his named Maternidad, a detail which he himself considers the most important part of the painting and which nobody besides him and the woman pay attention to.
He spends the next few months obsessing over her, thinking of ways to find her in the immensity of Buenos Aires, and fantasizing about what to say to her.
Ultimately, he winds up bumping into her on an elevator, and succeeds in making contact. Her name is María Iribarne Hunter, and she's married to a blind man named Allende and lives on Posadas street in the northern part of the city. Castel keeps seeing her for months, and much is made of his obsessive interrogations of her to satisfy his crazed mind, his twisted logic leading him to all sorts of bizarre patterns of thought and conclusions, and his interior monologue (in fact, the style of the story is first-person).
Allende has a cousin named Hunter who lives on a country estancia near Mar del Plata, and much of the action takes place around or in reference to that estate.
Ultimately, Castel's paranoia and mistrust lead him to the conclusion that María is in fact a full-time whore who cheats on her husband not only with him, but also with Hunter and almost any other man. In a fit of rage, he drives out to the estancia and stabs her to death.